Hot dip metal coating method



p 1953 .1. T. MAYHEW HOT DIP METAL COATING nmaon Filed Nov. 22, 1960FIG] FIGZ

I m d INVENTOR.

JOHN T MAYHEW ATTORNEY United States Patent 3,104,981 HGT DIP METALCOATING METHGD John T. Mayhew, Toronto, Ohio, assignor to National SteelCorporation, a corporation of Delaware Filed Nov. 22, 1960, Ser- No.71,009 2 Claims. (iii. 117-46) The invention is concerned generally withhot-dip coating and more particularly with a method for producing aneven coating on heavy coated flat rolled products.

In the hot-dip coating of flat rolled products, such as hot-dipgalvanizing, considerable difiiculty has been experienced with drapingof the coating when a heavy coating is applied. For example, in thecontinuous hotdip galvanizing wherein a coating of about 1.5 ounces persquare foot of the product is applied, the molten metal will run atfrequent intervals along the surface of the product. Visually therunning or draping of the molten galvanize coating has the sameappearance as that occurring when a too heavy brush stroke of wet paintis applied to a wall. This draping of the molten coating appearsfrequently, usuflly at less than two foot intervals longitudinally andless than 1 foot intervals laterally along the surface of the product.In solidifying, the run or drape presents an uneven, unattractivesurface.

Uneven galvanize coating has other disadvantages in addition to itsappearance. The draping actually removes coating from the product sothat the valleys formed are below coating specifications. Correction ofdraping has long been a problem in the hot-dip galvanizing industry. Allpreviously known solutions employed a process known as puddling. In thisprocess, streams of compressed air were blown vertically upward onto thesurface of the flat rolled product in a direction opposite to thedirection of drape. If the air streams were properly positioned and atfrequent enough intervals, a fair job could be done in minimizing thedraping effect. Actually only the heaviest draping was smoothed over andthe coating was moved by the air blasts so that the problem of unevencoating of the product still remained. Poor puddling resulted insplattering of the metal and wasted product. Puddling, even at itshighest efiiciency, is not adequate to the job as it is merely aremedial process rather than a preventative process. A significant partof the invention was the discovery and analysis of the source of theproblem; a novel preventative process resulted therefrom.

In explaining the invention reference will be had to the accompanyingdrawings in which:

FIG. 1 is a side view of hot-dip coating apparatus;

FIG. 2 is a frontal view of the apparatus of FIG. 1;

FIG. 3 is an enlarged view of a portion of hot-dip coating apparatus;and

FIG. 4 is an enlarged view of a portion of coating apparatus employingthe invention.

Referring to FIG. 1, in hot-dip galvanizing a flat rolled product isintroduced into a molten bath 11 through a controlled atmosphere chute12. The flat rolled product is at a temperature above that of the moltenbath and maintains the bath molten. The bath is normally zinc base withaluminum additions to suppress zinc-iron a1- loying.

The product travels downwardly around submerged roll 14- and thenupwardly toward exit means which also function as coating control means.Coating control is effected by coating rolls -15 and 16; with the levelof the molten bath 11, the pressure between the rolls 15 and 16, and thenumber of circumferential grooves present on the rolls determining thecoating thickness.

After exit from the molten bath through the coating rolls 15 and 16 thecoated product 17 travels upwardly into cooling towers (not shown).Normally the coating begins to solidify at about 8 to 15 feet above thenip of 3,164,981 Patented Sept. 24, 1963 "ice the coating rolls, thedistance varying with the temperature employed, the gauge of the basemetal being coated, and the coating thickness. Before solidificationsome galvanized coatings above one ounce per square foot of product, andin all cases where galvanized coatings above about 1.5 ounces per squarefoot of product are applied, the draping eifect described earlier takesplace.

The invention includes the discovery of causes for the aforedescribeddraping of molten coatings in hot-dip coating practice. As the coatedflat rolled product 17 moves upwardly through the coating rolls 15 and16, molten metal accumulates above the nip 21 of the rolls 15 and 16 increvices 22 and 23, existing on either side of the product between thecoated flat rolled product 17 and the adjacent coating roll. The metalin the crevices '22 and 23 takes on a rotary motion because of themovement of the coated product 17 and the coating rolls 15 and 16 sothat beads of metal 26 and 27 are formed in each crevice. The bead ofmetal in each crevice due to the relative motion between it and thecoated product 17 picks up additional metal from the coated product andgrows with a snowballing efiect. This pick up of metal from the surfaceof coated product is believed to be caused by the surface tensionbetween the molten coating metal on coated product 17 and metal in thecrevices 22 and 23 respectively. At periodic intervals, as metalaccumulates along crevices 22 and 23 the cohesive force of a bead ofmetal is. overcome and a heavy blob of metal is deposited on theproduct. As the product 17 travels upwardly the heavy blobs of metalstart to drape and pull further coating from the product above thedrape.

In accordance with the invention the build-up of metal in the crevices22 and 23 and the deposition of such metal on the coated product 17 isprevented. This buildup and deposition of; metal is prevented by blowinga gas downwardly into the crevices between the coated product 17 and thecoating rolls 15 and 16. By properly selecting the direction of flow ofgas into the crevices 22 and 23 the draping of molten coating iseliminated entirely. It is believed the gas reduces the surface tensionbetween the beads of metal 26 and 27 and the coating on the prodnot 17allowing the metal to flatten in the crevice as shown in FIG. 4.

When coke oven gas is blown into the crevices it is ignited by thetemperatures existing around the coating pot. It has been observed,however, that the draping effect is discontinued immediately upon theflow of the gas, prior to the ignition thereof, so that a flame is notnecessary to the operation of the invention.

Referring to FIGS. 2 and 4 a gas is introduced into the crevices throughthe pipes 311 and 31 and flows with a high volume and low pressure. Thegas after contact in the crevices flows outwardly toward the lateraledges of the coated product 17 and the coating rolls 15 and 16. By theuse of a high volume-low pressure gas splattering of the metal isavoided, and the coating remains evenly distributed on the coated flatrolled product -17.

Suitable gases include compressed air, or inert gases such as nitrogen.It is preferred that the gas be hot, either heated or in the form of aflame to prevent chilling of the coating rolls 15 and 16. Therefore,when heated gas is used a temperature approaching that of the coatingbath 11 is employed to prevent chilling of galvanizing material on therolls and the fouling thereof. When coke oven gas, blast furnace gas, ortheir equivalent is used a normal yellow, low-temperature flame issatisfactory as undue heating of the rolls or coating material is notdesired. It is emphasized that having heat present in the gas'preventschilling of the:coating rolls.

Under normal circumstances a single gas outlet located 7 centrally ofthe flat rolled product is sufficient. It is important that thedirection of gas flow be substantially 3 ertically downward onto themolten metal in the crevices 22 and 23; such a path has the efifect offlattening the molten metal in the crevices as shown in FIG. 4. Normallythis flow path of the gas will be substantially parallel to the path ofthe exiting flat rolled product 1'7.

In a specific embodiment of the invention employing coke oven gas thefollowing values were measured:

Gas pressure, in pipe lbs./in. 2-3 Gas pressure, between pipe andcrevice lb./in. Gas volume ft. /hr 306-500 Temperature, thermo-coupleafter 35 minutes in thefiame F 1000 Summarizing the invention, the causeof molten coating drape in hot-dip, heavy coating of fiat rolled producthas been discovered and analyzed and a method devised for preventingdraping in which a gas is blown downwardly into a crevice between theexiting fiat rolled product and the coating control means along a pathwhich is substantially parallel to the path of the exiting flat rolledproduct.

What is claimed is:

1. In continuous strip-steel hot-dip metal coating wherein a heavygalvanize coating weight is applied such that draping of the moltengalvanize coating on the strip would normally occur, a method forpreventing such draping of the molten coating comprising the steps ofapplying a galvanize coating to the continuous strip steel by passingthe strip upwardly through a molten metal bath,

controlling the weight of the galvanize coating to be at least one andone-half ounces'per square foot by passing the coated strip betweencoating control rolls located so as to deliver the strip upwardly fromthe molten bath, the exiting strip and the coating control rolls forminga crevice located on a face of the strip and containing a bead ofmoltenmetal, and preventing draping of the molten metal coating on the stripafter passage of the strip through the Coating control rolls withoutchanging the coating weight by contacting the surface of the head ofmolten metal with an ignited combustible gas along substantially theentire length of the bead, with the direction and velocity of theburning gas being such that the gas exerts substantially no force on themajor portion of the length of the bead.

2. In continuous strip-steel hot-dip metal coating wherein a heavygalvanize coating weight is applied such that draping of the moltengalvanize coating on the strip would normally occur, a method forpreventing such the *ing of the molten coating comprising the steps of:

applying a galvanize coating to the continuous strip steel by passingthe strip upwardly through a molten metal bath, 7 V controlling theweight of the galvanize coating to be at least one and one-half ouncesper square foot by passing the coated strip between coating controlrolls located so as to deliver the strip upwardly from the molten bath,the exiting strip and the coating control rolls forming a crevicelocated on a face of the strip and containing a bead of molten metal,and I preventing draping of the molten metal coating on the strip afterpassage of the strip through the coating control rolls Without changingthe-coating weight by contacting the surface of the bead of molten metalwith an ignited combustible gas along substantially the entire length ofthe bead, with the direction of how of the burning gasbeingsubstantially parallel to the length of the bead along the majorportion of the length of the bead whereby the burning gas exertssubstantially no force on the major portion of the length of the bead.

References Cited in the file of this patent UNITED STATES PATENTSWhitley et al .July 18, 1961

1. IN CONTINUOUS STRIP-STEEL HOT-DIP METAL COATING WHEREIN A HEAVYGALVANIZED COATING WEIGHT IS APPLIED SUCH THAT DRAPING OF THE MOLTENGALVANIZE COATING ON THE STRIP WOULD NORMALLY OCCUR, A METHOD FORPREVENTING SUCH DRAPING OF THE MOLTEN COATING COMPRISING THE STEPS OF:APPLYING A GALVANIZE COATING TO THE CONTINUOUS STRIP STEEL BY PASSINGTHE STRIP UPWARDLY THROUGH A MOLTEN METAL BATH, CONTROLLING THE WEIGHTOF THE GALVANIZE COATING TO BE AT LEAST ONE AND ONE-HALF OUNCES PERSQUARE FOOT BY PASSING THE COATED STRIP BETWEEN COATING CONTROL ROLLSLOCATED SO AS TO DELIVER THE STRIP UPWARDLY FROM THE MOLTEN BATH, THEEXITING STRIP AND THE COATING CONTROL ROLLS FORMING A CERVICE LOCATED ONA FACE OF THE STRIP AND CONTAINING A BEAD OF MOLTEN METAL, ANDPREVENTING DRAPING OF THE MOLTEN METAL COATING ON THE STRIP AFTERPASSAGE OF THE STRIP THROUGH THE COATING CONTROL ROLLS WITHOUT CHANGINGTHE COATING WEIGHT BY CONTACTING THE SURFACE OF THE BEAD OF MOLTEN METALWITH AN IGNITED COMBUSTIBLE GAS ALONG SUBSTANTIALLY THE ENTIRE LENGTH OFTHE BEAD, WITH THE DIRECTION AND VELOCITY OF THE BURNING GAS BEING SUCHTHAT THE GAS EXERTS SUBSTANTIALLY NO FORCE ON THE MAJOR PORTION OF THELENGTH OF THE BEAD.